Ink-jet printing system jets and lands minute droplets of ink from ink-jet heads on a base material to be printed, thereby printing by forming an image or characters on the base material. This ink-jet printing system does not require a printing plate for printing. Electrographic system is a widely known as a typical printing system requiring no printing plate. However, the ink-jet printing system is superior to the electrographic system in the initial cost of the apparatus, the running cost of printing, the apparatus size, and high-speed printability.
The ink-jet printing system typically uses ink-jet ink of active energy ray-curable type. This ink mainly contains polymerizable compounds such as a reactive monomer and oligomer, and a polymerization initiator such as a photo-radical generator and a photoacid generator. In the printing on this printing system, the ink is landed on a base material to be printed, and then active energy rays are applied to the ink to cause polymerization reaction of the ink components, thereby forming an image and characters.
In recent years, with the improvement of performance of ink-jet heads, the application of the ink-jet printing system to the existing printing market using offset printing and other systems is expected. In the existing printing market, productivity is very important. However, the multi-pass printing system is used in the sign market, and this printing system is insufficient to achieve the desired productivity. Therefore, in the printing market, in order to achieve productivity using the ink-jet printing system which cannot be obtained by the multi-pass printing system, the use of the single-passing system which allows high-speed printing is desired.
The single-passing system often uses fixed ink-jet heads, and thus requires head cleaning. Typically, head cleaning is carried out once per every several hours, and the frequency is low. Therefore, the single-passing system requires the designing of ink which will not cause deflection or ejection failure of ink. In particular, when titanium oxide is used as the white pigment of a white ink, specific gravity of the pigment in the ink is high. Therefore, when the apparatus is ceased for a while, the pigment tends to sediment in the nozzles. Furthermore, pigment sedimentation tends to cause problems such as ink ejection failure (nozzle void), so that improvement is desired.
Patent Literatures 1 to 3 disclose the methods for preventing sedimentation of titanium oxide through the surface treatment of titanium oxide used as the pigment, and/or the use of a specific pigment dispersant. Prevention of sedimentation of titanium oxide likely improves ejection stability. However, by any of these disclosed methods, it is difficult to achieve sufficient ejection stability over a long time which allows favorable use of inks on the single-passing system.
Patent Literature 4 discloses a method of improving pigment sedimentation by using hollow particles in place of titanium oxide as the white pigment. The hollow particles have a lower specific gravity than titanium oxide, so that the hollow particles resist sedimentation even in a low viscosity ink. However, the hollow particles tend to have lower hiding property in comparison with titanium oxide. Therefore, in order to achieve a sufficient printing concentration by printing on the single-passing system, for example, the printing concentration must be improved by using multiple heads, which may cause a problem of upsizing of the apparatus.